linux-kernel - Re: [PATCH v9] kallsyms: Add self-test facility

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Date:   Fri, 16 Dec 2022 12:38:04 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Andreas Schwab' <schwab@...ux-m68k.org>,
        "'Leizhen (ThunderTown)'" <thunder.leizhen@...wei.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        "Joe Lawrence" <joe.lawrence@...hat.com>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Luis Chamberlain" <mcgrof@...nel.org>,
        "linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH v9] kallsyms: Add self-test facility

On Fri, 16 Dec 2022 12:19:47 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> I assumed that "memory" was for memory unrelated to the input constraints.

Well, it looks like you do need a "memory" barrier.

  https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

"memory"

      The "memory" clobber tells the compiler that the assembly code
      performs memory reads or writes to items other than those listed in
      the input and output operands (for example, accessing the memory
      pointed to by one of the input parameters). To ensure memory contains
      correct values, GCC may need to flush specific register values to
      memory before executing the asm. Further, the compiler does not
      assume that any values read from memory before an asm remain
      unchanged after that asm; it reloads them as needed. Using the
      "memory" clobber effectively forms a read/write memory barrier for
      the compiler.

As the "(for example, accessing the memory pointed to by one of the input
parameters)" is exactly this case.

-- Steve